If you want to boost your popularity rating with your spouse and kids, put in an extra bathroom. It’ll eliminate those morning lineups, keep the kids and their friends from tromping through the living room with muddy feet, and please older folks because they won’t have to climb the stairs.

This project ranks so high on homeowner surveys that builders now put extra bathrooms in new homes as standard equipment. But if you own an older home, you can probably use another.

In this article we’ll show you how to plumb a “half bath,” so called because it has a sink and a toilet but no shower or tub. It’s small enough so you can usually squeeze it into your current floor plan. See “How to Find Space,” p. 32.

This article is an updated and completely rewritten version of a popular plumbing feature we ran in Sept. ’93.

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Building an entirely new bathroom is a big project involving carpentry, plumbing, electrical, drywall and usually ceramic tiling skills. In this article, we’ll focus on the most difficult part: the plumbing. Plan to spend three or four days on the plumbing and about two weeks for the entire bath. Remember to apply for a permit to make sure your work is inspected and approved. On p. 42, we list detailed TFH articles on tiling, drywall, setting a toilet, and electrical wiring.

Step 1: Measure and sketch your proposed drain route; shorter is better

Plumbers claim that you can plumb a bathroom no matter where you put it. But the farther you locate the bathroom from your home’s main waste line, the more problems you’ll encounter. The main waste line is a 3- or 4-in. dia. pipe called a “soil stack” when it runs vertically inside the house, and a “building drain” when it runs horizontally underground. On the main floor, it’ll be enclosed inside finished walls, but you can usually locate it by looking in the attic or basement or on top of the roof, where it exits to open air. You’ll have to attach at minimum a 3-in. dia. waste line from your new toilet to this main waste line. The shorter the run, the better, so you don’t have to break into as many walls, floors and ceilings. In extreme cases, you may have to box in a portion of a ceiling for a drainpipe. The main drain lines can be more difficult to find in a slab-built house because they run under the concrete. If you can’t locate them, call in a licensed plumber to help with this crucial part of the planning.

TIP:

The closer you locate the bathroom to your home’s main waste line, the fewer problems you’ll encounter.

How to find space

Remember the old-time half bath that was so cramped you had to sit on the toilet sideways? No longer. Most building codes now have minimum space requirements for toilets: 30 in. in width and 24 in. of open space to the front. Fig. A shows the approximate minimum room size needed to fit standard sinks and toilets. However, you can shrink the room more if you choose smaller fixtures. To find them, check the catalogs at a full-service plumbing store or a home center, or consult a bathroom designer.

Designers look to closets, pantries and portions of bedrooms as the most likely places where they can “steal” space. However, you don’t want the door opening to kitchens, dining rooms or main living areas. Basements are ideal locations too.

You might have to shift a wall or two to get the right dimensions. “Bearing walls,” which support the weight of your home, are difficult to remove. (See “How to Remove a Bearing Wall” in “For More Information,” p. 43.) Other walls can be shifted with little trouble unless they have heating ducts and plumbing runs inside them.

Fig. A Opposite walls

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Fig. B Same wall

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Using the guidelines in Figs. A and B for space requirements, sketch the floor plan of your proposed half bath on paper and draw in the proposed route for your drains (Fig. C, p. 37). Then add the vents. Vents are pipes that equalize pressure in your drain system to ensure that the traps remain filled with water and keep sewer gas from stinking up your house. Each fixture (sink, toilet, shower, etc.) must have a trap and every trap must have a vent. The vent system runs upward and either connects to the main vent system or runs up through the roof separately.

Follow the rules for drains and vents on p. 40 so you can accurately plan their routes. Label the pipe sizes on your sketch, along with the necessary fittings—ells (90-degree elbows), tees, etc. Show this plan to your local building inspector, who will check the sizes of your pipes and fittings (codes vary from region to region) and issue a permit. The inspector will also tell you how to pressure-test the drain and vent system for leaks after you install the pipe. We’ll walk you through this important testing procedure on p. 40. Testing the water supply system is easier. If it leaks when you turn it on, you goofed!

Step 2: Carve out your space and frame the half bath

Begin by cutting away the carpet or prying up other finished flooring from the half bath area. Keep the subfloor (the bottom layer of flooring) intact and frame the new bath walls with 2x4s. If you have to frame a wall that contains the 3-in. or 4-in. waste line, use 2x6s. Remember that if you remove a bearing wall, you must replace it with a beam to support the weight it was carrying.

The open space in your half bath might be too small for a normal in-swinging door. If an outward swing is too awkward, consider a pocket door, which slides back into the wall. (See “Install a Pocket Door,” Nov./Dec. ’92, p. 43. To order a copy of the article, see p. 116.)

If the toilet drain is over a joist (floor framing), try shifting the toilet. Cutting a floor framing member can seriously weaken the floor. If it’s necessary to do so, consult with a local building inspector or a professional carpenter for an approved method.

TIP:

Begin installing drains from the most distant point and work toward the existing system.

With the walls framed and open, install the plumbing, electrical circuits and exhaust fan (Fig. C).

Step 3: Install the drain system; it’s all downhill

Photos 1 – 3 illustrate how to run the toilet drain, a 3-in. pipe that connects the toilet flange to the 4-in. soil stack (Fig. C). We show white PVC plastic, type “Schedule 40,” but some regions may require black ABS plastic. Use the specified glue to cement each type.

Begin by cutting and “dry-fitting” (no glue) the system, working from the point most distant from the existing main drain. To establish a solid end point—the toilet flange in this case—screw tile backer board to the subfloor and slide two tiles under the flange to get the height exactly right (Photo 2) before screwing the flange down. Then cut the plastic pipe with a handsaw or power miter box and assemble the fittings. When you’re finished with the run, disassemble everything, prime all the fittings with purple primer (for PVC) and glue them together. Spread the cement on each pipe and fitting and shove them together, giving each a slight twist to help seat them fully. Hold each joint together firmly until the glue sets, about 30 seconds.

CAUTION: Wear a respirator with an organic vapor cartridge when working in poorly ventilated areas. Both the PVC primer and the cement contain powerful solvents.

The dry-fitting technique has some pitfalls. Plastic pipe doesn’t always seat tightly in the fittings until you apply the cement. One section cut too short can throw several other sections off. And dry-fit joints sometimes stick together. You might have to tap them with a hammer and wood block to get them apart.

TIP:

Make the drain holes that you cut through the floor and 2x4s slightly larger than the pipes. If a pipe fits tightly, it’ll squeak whenever it expands and contracts.

With experience, you’ll find it’s easier to glue the pipes and fittings as you go rather than to dry-fit them. In any case, mistakes are easy to correct. Simply cut off the pipe and add another. Pipe and most fittings are cheap.

To ensure good draining and venting, follow the guidelines outlined in “Drain and Vent Rules,” p. 40. Meeting the minimum distance requirements from trap to vent can be challenging. We had to angle our toilet drain toward the side wall to meet the distance rule (Fig. C). Note in the rules that vent sizes and their distance from the trap vary with the trap size.

Fig. C Plumbing and Electrical Details

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Assemble the sink drain, working back from the “wye” in the toilet drain (Photo 5). Don’t connect the drains to the main soil stack until you’ve pressure-tested the system.

Step 4: Run the vents from the drains to the roof

Mark and cut the holes in the wall plates for the 2-in. vent (Photo 4), and glue the vent pipe into the sanitary tee in the 3-in. drain (Fig. C). You can cut these holes with a large hole saw, but it’s difficult and even a bit dangerous (the saw can catch and jerk the drill out of your hands). Photo 4 illustrates a cruder, but effective, method using common tools.

A vent must rise vertically (defined as at least a 45-degree angle to vertical) within a certain distance of the trap it serves, depending on the diameter of the drainpipe. (See “Drain and Vent Rules,” p. 40.) We had to angle our 3-in. toilet drain under a nearby wall to provide a place to run the vent within the required distance. Other vents, like our sink vent, can run directly up from the point where the drain enters the wall. You can also run vents almost horizontally, but only after the vent is 6 in. above the “spill line” of the sink or tub or whatever other fixture you’re venting. Since we wanted to install a recessed medicine cabinet only about 5 in. above the spill line, we had to offset the vent with a couple of 45-degree elbows rather than 90-degree elbows (Fig. C). Even when the vent runs horizontally, allow a slight slope back toward the drain so that water that gets into the vent will drain out.

Where to get technical help

Some inspectors will be more helpful than others. While all inspectors will explain the local code, it isn’t their job to teach you how to do the work. However, if you buy your supplies from a full-service plumbing store, the sales staff will usually give you sound advice. You can also hire a professional plumber to help you design the system and supply installation tips. In addition, see the articles listed on p. 43.

Whenever plastic or copper pipe passes within 1-1/4 in. of the surface of a plate or stud, cover the area with a 1/16-in. steel protective plate (about 25¢); see Photo 5. This plate will stop or deflect drywall screws and trim nails that could puncture the pipe.

Run both the sink and toilet vents into the attic and connect them to nearby vents from other plumbing fixtures. If you have enough attic space, you can even connect them to the 3- or 4-in. main stack. Otherwise, climb onto your roof, cut a hole for the new vent and extend the 2-in. vent at least 12 in. above the roof (Photo 6). Drop a special vent flashing ($5) over the vent and weave the shingles over it to make it leakproof.

If you can’t vent your bathroom fan through the side wall or roof overhang, now’s the time to run it up through the roof too (Photo 6).

CAUTION: Working on a roof is dangerous. Wear safety equipment and stay away from the roof edges. (See March ’96, p. 60, for more details and p. 116 for where to send for it.)

Drain and vent rules

(Plumbing codes vary. Confirm your plans with a local plumbing inspector.)

• Position a 2-in. vent to rise vertically from the drain within 4 ft. of the 3-in. toilet flange (the trap is inside the toilet).

• Position a 1-1/2 in. vent to rise vertically within 42 in. of a 1-1/2 in. trap.

• Maintain a uniform slope of at least 1/4 in. per foot on all drain lines (Photo 3).

• Run the vent vertically to a point 6 in. above the “spill line” (overflow level) of the fixture.

• Slope vents back toward the drain.

• Use plastic hangers to support PVC every 32 in.

Step 5: Solder the supply lines; you can work them in almost anywhere

Running the hot and cold water lines is easier than installing the drains and vents. You can snake these lines around to fit just about anywhere. However, it’s tricky to solder copper joints in tight spots. Use a metal plate to keep wood from igniting in close quarters, or buy a special flame-blocking cloth ($12 in home centers).

To begin, turn off the water at the main valve where it enters your home, and drain the lines by opening the lowest and highest hot and cold faucets. Then cut into existing hot and cold lines where they’re readily accessible and run 1/2-in. copper pipe (or CPVC plastic if it’s allowed) to the new toilet and sink (Fig. C).

TIP : If running your new lines looks complicated, install valves ($4 each) at the take-off point. Then you can turn the water back on and keep the family happy while you work.

Follow these two rules:

• Half-inch pipe can serve only two or three fixtures (depending on the code in your region). You might have to tap into a 3/4-in. supply line for the cold water for your new toilet and sink.

• Support copper every 6 ft. (Photo 7) and CPVC every 32 in. with straps.

Standard wall locations for the hot and cold water for sinks are 21 in. up from the floor and 4 in. to the left and right of center (Fig. C). The standard wall location for the toilet supply is 6 in. up from the floor and 6 in. to the left of center as you face the toilet. However, locations might vary for pedestal sinks and special toilets, so read the directions that come with the fixture.

Anchor the lines where they stub out from the wall with a strap nailed to a stud or block. Otherwise, anchor them to a special copper plate ($2 from plumbing suppliers) that you nail to the studs (Photo 8). Finally, solder caps on the ends of the stubouts to seal the system. Then turn on the water and look for leaks.

Step 6: Test the drain and vent system so you know the system is leakproof

A leak in the drain and vent system will allow wastewater or dangerous sewer gas to invade your home. Most plumbers test the system with a small air compressor and a pressure gauge assembly similar to the one we show in Photo 9. The parts cost about $30, so rent it from a plumbing specialty store or a plumber if you can. Insert airtight plugs (from $2 to $6 each) in all open ends of the system—the vent on the roof, the toilet flange and, as shown in Photo 10, the drain. Pump it up to 5 lbs. of pressure and watch the gauge to see if the pressure holds for 15 minutes. If it doesn’t, increase the pressure again to 5 lbs. and dab a 50/50 mixture of dishwashing soap and water around the plugs first (to make sure they’re tight) and then on the joints. A leak will cause the solution to bubble. Saw out the bad joints and glue in new fittings.

TIP:

Don’t flush the toilet on the second floor while the stack is open! Or run any water, for that matter.

Some inspectors permit a water test, in which you plug all the lower openings and fill the system from the top, usually through the vent on the roof. If the water level on top drops within 15 minutes, look for the leak(s) at the joints below.

Step 7: Crack into the soil stack to make the final drain connection

With the system tested, connect the drain to the soil stack. If the stack is plastic, simply saw out a section to fit the proper-sized tee fitting, and connect everything with approved transition couplings (Photo 11). You usually can’t glue in fittings with hubs (the wide part of the fitting that the pipe slides into), because the main stack won’t move up and down.

Older homes have cast iron stacks that you have to “snap” with a special soil pipe cutter ($25 a day to rent) when you’re working in close quarters (Photo 10). Follow this procedure:

• Bolt a “riser clamp” ($8) to the stack and fasten 2x4s under each end to support it (Photo 10).

• Measure the new cast iron tee and mark the stack for cutting, adding 1/4 in. to the measurement to make sure the tee slips in.

• Wrap the cutter chain around the pipe so the cutting wheels rest on the mark. This tool weighs about 50 lbs., so you might need a helper here.

• Lever the tool until the pipe snaps. (Ask the clerk at the rental yard to explain how to operate the tool.)

• Snap the pipe at the second mark as well, then knock out the section with a hammer and block of wood.

• Insert the new fitting and secure it with the clamps we show in Photo 11. We used a torque wrench ($25) set at 60 in.-lbs. to tighten the band clamps. At this torque, the rubber gaskets should be crimped firmly, but not so tightly that they bulge out from the pressure.

For More Information

• “Bathroom Makeover,” May ’94, p. 72. (Install new vanity, sink and mirror.)

• “How to Install a Bath Fan,” Feb. ’96, p. 50.

• “Better Bathroom Lighting,” Nov. ’96, p. 74.

• “Plumbing a New Basement Bath,” Feb. ’98, p. 65.

• “How to Replace a Toilet,” Dec./Jan.’99, p. 67.

• “Tips for Better Tiling,” Nov. ’97, p. 36.

• “How Pros Hang Drywall,” Nov. ’98, p. 61.

• “Soldering Copper,” May ’94, p. 64.

• “Space-Saving Pocket Doors,” Feb. ’95, p. 44.

For information on how to order back issues, copies of articles or the Five-Year Index, please see p. 116.

Now finish the bath; you’re more than halfway there

With the plumbing roughed in, proceed with the electrical work. Once the plumbing, electrical and building inspectors approve your rough-in work, close up the wall with drywall, install the tile, hang the door and complete the other finish work. Please refer to the articles listed below for complete how-to information.

The final steps are to set the toilet and sink (Photos 12 and 13). Turn off the water at your home’s main valve again, drain the system and then cut off the capped ends of the copper stubouts with a tube cutter ($5). Install escutcheon plates and shutoff valves on each one. Then assemble and install the fixtures. Afterward, turn the water back on and check for leaks.

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1 BEGIN LAYING DRAINS from the most remote location and work toward the existing system. Position the toilet flange first and cut a hole for it through the subfloor with a jigsaw. Center it 12 in. out from the finished wall, allowing space for drywall and tile in this case.

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2 ANCHOR THE FLANGE with screws at the finish floor height to provide a solid point for drainpipe assembly. Use purple PVC primer before gluing PVC pipe and fittings.

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3 “DRY-FIT” THE DRAIN LINE (no glue) to position all the fittings and pipe lengths. Then disassemble it and glue it together with special PVC cement. Slope the drain a minimum of 1/4 in. per foot and support the pipe every 32 in.

4 CUT HOLES in the wall plates for the 2-in. vent. One method is to drill a series of holes with a 3/4-in. bit and cut out the center with a keyhole saw.

5 RUN THE SINK DRAIN beginning from the “wye” in the 3-in. drain. Center the sink drain along the wall (for a centered sink) and glue the drain assembly when the pieces fit. Wear an organic vapor respirator to avoid breathing PVC cement fumes.

CAUTION: Wear a respirator with an organic vapor cartridge when working in poorly ventilated areas. Both the PVC primer and the cement contain powerful solvents.

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6 EXTEND THE VENTS up through the attic and connect them before running the 2-in. vent through the roof. Drop special vent flashing over the vent pipe and shingle over it so it won’t leak. Add a vent for the bath fan if it exhausts through the roof.

7 CUT AND ASSEMBLE the 1/2-in. hot and cold water supply lines. Solder copper and support it every 6 ft. Temporarily protect nearby flammable material with a steel plate while soldering. Keep a fire extinguisher or bucket of water nearby. With the system off, you won’t have water available in an emergency.

8 SUPPORT THE SUPPLY LINES where they stub out from the wall with wood blocks and straps or this special copper strap that you nail to the studs. Solder the pipes to this special strap.

9 RENT A PRESSURE GAUGE ASSEMBLY and compressor and pressure-test the drain/vent system for leaks. With the open ends of the system temporarily plugged, it should hold 5 lbs. of pressure for 15 minutes.

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10 SNAP THE CAST IRON SOIL STACK with a rented soil pipe cutter and remove a section for the new fitting. Bolt a special riser clamp to the stack first and support it with 2x4s to keep the heavy stack from dropping down.

11 SLIP THE RUBBER/ STEEL COUPLINGS onto the cut pipe ends first, insert the cast iron fitting and work the couplings over the joints. A torque wrench keeps you from overtightening the band clamps.

12 ATTACH A SHUTOFF VALVE to the supply line. Then set the wax ring and gently lower the toilet over the hold-down bolts in the toilet flange. Connect a 3/8-in. supply tube to the valve and to the toilet tank.

13 ATTACH THE SHUTOFF VALVES to the hot and cold supply lines. Then assemble the sink and faucet and bolt the sink to the wall. Connect 3/8-in. supply tubes to the valves and a 1-1/4 in. P-trap to the drain.

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Art Direction • MARCIA WILLISTON

Photography • MIKE KRIVIT

Illustrations • DON MANNES

Consultants • CHARLIE AVOLES and DARWIN BAACK, MASTER PLUMBERS